1. Field of the Invention
This invention relates generally to systems and methods for performing spinal fixation and, in particular, to interbody spacer devices.
2. Description of the Related Art
Advancing age, as well as injury, can lead to degenerative changes in the bones, discs, joints and ligaments of the spine, producing pain and instability. Under certain circumstances, alleviation of the problems can be provided by performing spinal fusion. Spinal fusion is a surgical technique where two or more vertebrae of the spinal column are fused together to eliminate the motion between the fused vertebrae. Spinal fusion is used to treat conditions where the spine exhibits instability. Spine instability may result from causes such as fracture, scoliosis and spondylolisthesis, where one or more vertebrae move in a forward direction relative to the other vertebrae. Spinal fusion with discectomy is also performed for herniations of the discs. This surgery involves removal of the affected disc and fusion of the adjacent vertebrae. Traditionally, bone grafts have been used to fuse the vertebrae, but various types of vertebral implants have also been used.
The use of bone plate and bone screw fixation systems for treating injuries to bones is well established. In most instances, a bone plate is positioned over and surrounding the bone injury area and secured to the bone. The bone plate is secured to the bone by bone screws or other similar fasteners inserted through holes in the bone plate and into the bone itself. The screws are tightened so that the bone plate holds the bone to be treated in place in order to insure proper healing. Early fixation devices tended to be applicable only to long bone injuries with only limited uses for lower lumbar spinal injuries and disorders. The use of plate/screw fixation systems later expanded, however, to include more uses for spinal injuries, including fusion of vertebrae including fixation devices for treating cervical vertebrae injuries. Notwithstanding the foregoing, there remains a need for improved methods and devices for treating spinal instability.
In existing spinal fusion implants there have also been problems with loosening and backing out of screws, especially in the cervical vertebrae where the screws can back out into the patient's throat area. Backout is the exhibited tendency of bone screws, which affix the bone plate to the bone(s), to loosen with respect to both the plate and bone, resulting in poor fixation, fusion and ultimately, healing. Essentially, this loosening of the bone screw causes the screw to work itself out of the bone into which it is implanted. This results in the bone plate being poorly fixed in place thus becoming devoid of its fixation capabilities. Usually, backout is caused by the chronic stress of bodily movement. While such loosening can be benign if limited in scope, it can lead to complications such as complete failure of the fixation device or incomplete bone fusion. Backout is particularly prevalent in areas of high bodily stress and movement, such as the spine.
To alleviate backout and its associated problems, current systems utilize secondary locking screws, locking collars or other secondary locking devices that hold the bone screws in place after deployment within the bone. In most systems, the bone screw is affixed into the bone through an opening in a bone plate. A locking device is then inserted into the bone screw. The locking device engages the head of the bone screw and is tightened which results in the bone screw being fixed in place within the bone, thus preventing backout.
While a locking screw or collar can alleviate backout, successful use of such locking device systems in the anterior cervical spine is particularly difficult because of anatomic constraints. Systems using multiple types of screws or collars to hold the bone screw in place are difficult to deploy within the confines of a small operating area available at the cervical spine. Furthermore, due to the small operating area, the surgeon implanting the device has great difficulty determining if the device is properly deployed. Any instrumentation implanted in the region must be minimally intrusive, yet have adequate strength to withstand the biomechanical loads to which it will be subjected. Thus, while current systems can help reduce instances of backout, their complex nature makes proper deployment very difficult and increases the chance of surgical error.
There is a need for an implant having a locking mechanism that can be easily and reliably locked in place to prevent the loosening of and backing out of the bone screws used to attach the implant to the vertebrae in the anterior aspect of the cervical, thoracic, and lumbar spine.
There is also a need for implants that can be implanted along a series of adjacent vertebrae. Implants adapted for use in the lumbar spine and the thoracic spine become much less usable in the cervical spine because of differences in anatomy. In the lumbar spine, the disc spaces are about 25% as tall as the vertebral bodies (i.e., the vertebral bodies are generally four times taller than the intervening disc space). In the cervical spine, the disc space can be 50% of the height of the vertebral bodies. The disc spaces in the cervical spine are generally not greater than 7 or 8 mm tall in most people.
Attachment of one fixation plate between two vertebrae often prevents the attachment of additional fixation plates between one of two vertebrae and an adjacent vertebra. This is especially true in the cervical spine region. The attachment of one fixation plate will reduce the surface area available to attach another fixation plate due to the small size of the cervical vertebrae and the minimum size required for each fixation plate. Because of this limitation in existing spinal fixation devices, treatment of spinal disorders may be suboptimal because disease in adjacent vertebrae cannot be treated adequately.